For determining a mass flow of a medium, especially a liquid flowing in a pipeline, often such measuring devices are used, which by means of a measurement transducer of vibration-type and a control and evaluation electronics connected thereto, create Coriolis forces in the medium and produce, derived from these forces, a measurement signal representing the mass flow.
Such Coriolis mass-flow meters have been known for a long time and have established themselves well in industrial application. Thus, for example in EP-A 317 340, U.S. Pat. Nos. 5,398,554, 5,476,013, 5,531,126,5,691,485, 5,705,754, 5,796,012, 5,945,609, 5,979,246,6,006,609, 6,397,685, 6,691,583, 6,840,109, WO-A 99 51 946, WO-A 99 40 394 or WO-A 00 14 485, Coriolis mass-flow meters are described, each with a measurement transducer of vibration-type. Each of the disclosed measurement transducers includes a single, straight measuring tube, which conveys the medium and vibrates during operation. Such measuring tube communicates with the pipeline via an inlet tube piece at its inlet end and an outlet tube piece at its outlet end. Each of the disclosed measurement transducers also includes: An exciter mechanism, which causes the measuring tube during operation to oscillate with bending oscillations in a tube plane; and a sensor arrangement for the point-wise registration of oscillations toward the inlet end and toward the outlet end of the measuring tube.
As is known, straight measuring tubes, when excited to bending oscillations of a first eigenoscillation form, effect Coriolis forces in the medium flowing through the measuring tube. These forces, in turn, lead to a superimposing, on the excited bending oscillations, of coplanar bending oscillations of a second form of eigenoscillation of higher and/or lower order, such that oscillations registered on the inlet and outlet ends of the measuring tube exhibit also a measurable phase difference dependent on the mass flow.
Usually the measuring tubes of such measurement transducers, for example those used in Coriolis mass-flow meters, are excited during operation to an instantaneous resonance frequency of the first eigenoscillation form, especially in the case of controlled to constant oscillation amplitude. Since this resonance frequency is especially also dependent on the instantaneous density of the medium, it is possible also by means of Coriolis mass-flow meters common in the market to measure, besides mass flow, also the density of flowing media.
An advantage of straight measuring tubes is that; for example, they can be emptied with a high degree of certainty completely in practically any installation orientation. Especially is this also true after a cleaning process performed inline. Additionally, such measuring tubes are, in comparison e.g. to omega-shaped or helically-shaped measuring tubes, essentially easier and accordingly more cost-favorably manufacturable. A further advantage of a straight measuring tube vibrating in the above described manner is, in comparison to bent measuring tubes, also to be seen e.g. in the fact that, during measurement operations via the measuring tube, practically no torsional oscillations are evoked in the connected pipeline.
On the other hand, a significant disadvantage of the above-described measurement transducers lies in the fact that, because of the alternating lateral deflections of the vibrating, single measuring tube, oscillating transverse forces of the same frequency can be caused to act on the pipeline. To this point in time, these transverse forces have only been able to be compensated to a very limited extent and only with a very high technical effort.
For improving the dynamic balance of the measurement transducer, especially for reducing transverse forces caused by the vibrating, single measuring tube acting at its inlet and outlet ends on the pipeline, the measurement transducers disclosed in EP-A 317 340, U.S. Pat. Nos. 5,398,554, 5,531,126, 5,691,485, 5,796,012, 5,979,246, 6,006,609, 6,397,685, 6,691,583, 6,840,109 or WO-A 00 14 485 include in each case a counteroscillator embodied as one or more pieces and affixed to the measuring tube on the inlet end, accompanied by the formation of a first coupling zone, and affixed to the measuring tube on the outlet end, accompanied by the formation of a second coupling zone. Such counteroscillators, which are implemented in the form of a beam or especially in tubular form or as a body pendulum aligned with the measuring tube, oscillate during operation out of phase with the measuring tube, especially with opposite phase, whereby the effect of the lateral transverse forces and/or transverse impulses brought about in each case by the measuring tube and the counteroscillator on the pipeline can be minimized and in some cases also completely suppressed.
Such measurement transducers with a single measuring tube and counteroscillator have proven themselves, especially in the case of those applications wherein the medium to be measured has an essentially constant density or a density which changes to only a very slight degree, thus, for those applications in which a net force acting on the attached pipeline, resulting from the transverse forces produced by the measuring tube and the counterforces produced by the counteroscillator, can initially be set, without more, assuredly to zero. In contrast, those measurement transducers, especially those disclosed in U.S. Pat. Nos. 5,531,126 or 5,969,265, in the case of applications with media having densities fluctuating over wide ranges, especially in the case of different media following one after the other, and even when to only a slight degree, exhibit practically the same disadvantage as measurement transducers without counteroscillators, since the above-mentioned net resultant forces are also dependent on the density of the medium and consequently can be different from zero to a considerable degree. Stated differently, also the inner part of the measurement transducer formed by at least the measuring tube and the counteroscillator is globally deflected during operation out of an assigned static rest position, due to density dependent imbalances and transverse forces associated therewith.
A possibility for reducing density dependent, transverse forces is described e.g. in U.S. Pat. Nos. 5,287,754, 5,705,754, 5,796,010 or 6,948,379. In the case of the measurement transducers shown there, the more middle, or high, frequency, oscillatory, transverse forces produced on the part of the vibrating, single measuring tube are kept away from the pipeline by means of an, in comparison to the measuring tube, very heavy counteroscillator, and, as required, a relatively soft coupling of the measuring tube to the pipeline, thus, in practical terms, by means of a mechanical low pass filter. A great disadvantage of such a measurement transducer is, among other things, however, that the counteroscillator mass required for achieving a sufficiently robust damping increases more than proportionately with the nominal diameter of the measuring tube. On the other hand, when using such a massive counteroscillator, one must assure that a minimum eigenfrequency of the measurement transducer (which becomes ever lower with increasing mass) still lies far from the likewise very low eigenfrequencies of the attached pipeline.
Different, farther-reaching possibilities for reduction of the density dependent, transverse forces are proposed e.g. in U.S. Pat. Nos. 5,979,246, 6,397,685, 6,691,583, 6,840,109, WO-A 99 40 394 or WO-A 00 14 485. In the case of the disclosed compensation mechanisms presented there, of essential concern is the expanding of a bandwidth, within which counteroscillator and offset sections are effective, by providing a suitable interaction of the individual components of the inner parts of the measurement transducers.
In particular, in U.S. Pat. No. 6,397,685, a measurement transducer of the aforementioned kind is disclosed, wherein a first balancing mass is provided as a mass balancing measure for the exciting oscillation and is connected, in the compensation cylinder's central plane perpendicular to the longitudinal axis, with the compensation cylinder. Then, second and third balancing masses are provided as a mass balancing measure for the Coriolis oscillation. The second and third balancing masses are embodied as end regions of the counteroscillator. In this manner, it is to be achieved that the oscillation system composed of the Coriolis measuring tube and the compensation cylinder is at least largely balanced with respect to mass both for the exciting oscillations of the Coriolis measuring tube as well as also for the Coriolis oscillations of the Coriolis measuring tube.
WO-A 00 14 485 also describes a measurement transducer of vibration-type for a medium flowing in a pipeline. In this case, provided are: An inlet end, first cantilever, which is coupled with the measuring tube in the region of a third coupling zone lying between the first and second coupling zones and which has a center of mass lying in the region of the measuring tube; and an outlet end, second cantilever, which is coupled with the measuring tube in the region of a fourth coupling zone lying between the first and second coupling zones and which has a center of mass lying in the region of the measuring tube. Each of the two cantilevers is provided for executing balancing oscillations, which are so developed that the transverse impulses are compensated, and, consequently, a center of mass of an inner part formed of measuring tube, exciter mechanism, sensor arrangement and the two cantilevers is held locationally fixed. Furthermore, WO-A 99 40 394 describes a measurement transducer of the aforementioned kind in which a first cantilever serving for producing counterforces acting against the transverse forces at the inlet ends, as well as a second cantilever serving for producing counterforces acting against the transverse forces on the outlet end are provided. In such case, the first cantilever is affixed both to the measuring tube in the region of the first coupling zone and also to the transducer housing at the inlet end, and the second cantilever is affixed both to the measuring tube in the region of the second coupling zone, as well as also to the transducer housing on the outlet end, such that the counterforces are so developed that the measuring tube is kept fixed in an assigned, static rest position, despite the produced transverse forces.
In the case of the aforementioned measurement transducers, including that described in U.S. Pat. No. 5,979,246, the problem of density dependent imbalances is solved, in principle, by matching an amplitude characteristic of the counteroscillator to the measuring tube oscillations, especially by amplitude dependently changeable spring stiffnesses of the counteroscillator initially, and/or during operation, in such a manner that the forces produced by measuring tube and counteroscillator essentially compensate one another.
Finally, in U.S. Pat. Nos. 6,691,583 and 6,840,109, measurement transducers are in each case disclosed, wherein, in each case, a first cantilever fixed in the region of the first coupling zone essentially rigidly to the measuring tube, counteroscillator and inlet tube piece and a second cantilever fixed in the region of the second coupling zone essentially rigidly to the measuring tube, counteroscillator, and outlet tube piece are provided.
The two cantilevers, especially ones arranged symmetrically about the middle of the measuring tube, serve here for producing in the inlet and outlet tube pieces bending moments dynamically, when the vibrating measuring tube together with the counteroscillator and, as a result, also the two coupling zones are shifted laterally from their respectively assigned, static, rest positions, with the bending moments being so developed that, in the deforming inlet tube piece and in the deforming outlet tube piece, impulses are produced, which are directed counter to the transverse impulses produced in the vibrating measuring tube. The two cantilevers are so embodied and so arranged for this purpose in the measurement transducer that a center of mass of the first cantilever lying in the region of the inlet tube piece and a center of mass of the second cantilever lying in the region of the outlet tube piece both remain essentially locationally fixed in a static rest position despite the fact that the measuring tube has been shifted laterally out of its assigned static rest position. The basic principal of this compensation mechanism is to transform lateral displacement movements of the vibrating measuring tube, which would otherwise act in a disturbing manner on the measurements and/or on the connected pipeline and which are superimposed on its primary deformations effecting the measurement effects, into counter deformations of the inlet and outlet tube pieces acting in a dynamically balancing manner in the measurement transducer, in order to largely eliminate the lateral deflection movements. By a suitable tuning of the inner part, the deformations of the inlet and outlet tube pieces can be so developed that the transverse impulses largely compensate one another, independently of the instantaneous oscillation amplitudes and/or frequencies of the measuring tube. In corresponding manner, it is thus possible also essentially to compensate the transverse forces produced by the vibrating measuring tube by means of transverse forces produced by the deforming inlet tube piece and the deforming outlet tube piece.
Further investigations have, however, shown that, although with the decoupling mechanism proposed in U.S. Pat. Nos. 6,691,583 and 6,840,109, in principle, very good results are achievable with respect to disturbance resistance, the configurations disclosed there, nevertheless, exhibit bandwidths, which are too small for the decoupling mechanism and, as a result, may not yield satisfactory results with respect to the disturbance resistance of the measurement transducer when it comes to certain applications, especially in those applications involving medium densities fluctuating within very wide ranges. However, it was furthermore also possible to determine that alone the application of a relatively heavy counteroscillator as proposed in U.S. Pat. Nos. 5,287,754, 5,705,754, 5,796,010 or 6,948,379 led to no clear improvements for the decoupling mechanism presented in U.S. Pat. Nos. 6,691,583 and 6,840,109.